IEEE section 802.11, which is hereby incorporated by reference in its entirety, defines several different standards for configuring wireless Ethernet networks and devices. For example, 802.11 standards that have been popularized include 802.11, 802.11(a), 802.11(b) and 802.11(g). According to these standards, wireless Ethernet network devices may be operated in either an infrastructure mode or an ad-hoc mode. In the infrastructure mode, the wireless network devices communicate with each other through an access point. In the ad-hoc mode, the wireless network devices (which are typically called mobile stations) communicate directly with each other and do not employ an access point. The term mobile station may not necessarily mean that a wireless network device that is actually mobile. For example, a desktop computer may incorporate a wireless network device and operate as a mobile station in an ad-hoc network.
Referring now to FIG. 1, a wireless network 12 that is shown operates in an ad-hoc mode as defined by IEEE section 802.11 and other future wireless standards. The wireless network 12 includes multiple mobile stations 14-1, 14-2, and 14-3 that transmit and receive wireless signals 16 directly with each other to form an ad-hoc network. The mobile stations 14-1, 14-2, and 14-3 do not continuously exchange data.
Since the mobile stations are often battery powered, it is important to minimize power consumption to preserve battery life. Therefore, some wireless devices implement a low power mode and an active mode. During the active mode, the wireless device transmits and/or receives data. During the low power mode, the wireless device shuts down components and/or alters operation to conserve power. Usually, the wireless device is not able to transmit or receive data during the lower power mode.
Wireless Ethernet network devices may be implemented by a system on chip (SOC) circuit that includes a baseband processor (BBP), a medium access controller (MAC) device, a host interface, and one or more processors. A host communicates with the wireless network device via the host interface. The SOC circuit may include a radio frequency (RF) transceiver or the RF transceiver may be located externally. The host interface may include a peripheral component interface (PCI), although other types of interfaces may be used. The processor(s) may be Advanced RISC Machine (ARM) processor(s), although other types of processors may be used.
The MAC device controls and selects different operating modes of the BBP and the RF transceiver. During operation, the MAC device instructs the BBP and the RF transceiver to transition to a low power mode to conserve power. The BBP and RF transceivers may include phase-locked loops (PLL), which are calibrated using a reference signal that is supplied by a crystal oscillator (XOSC). [0]The SOC may also include voltage regulators that provide regulated supply voltages to the system.
In an ad-hoc mode, the MAC device may instruct the BBP and the RF transceiver to transition to the low power mode when the mobile stations do not have data to exchange. Usually the voltage regulator in the BBP, the XOSC and PLL devices remain active and consume power during the low power mode.
In some conventional approaches, the operating voltage and/or the clock frequency are reduced during the low power mode while still allowing the system to operate at full capacity. In other conventional approaches, the way that functions are implemented is modified to reduce power consumption. For example, the device may lower a frequency of operation so that calculations take longer to complete.
In another approach, a wireless Ethernet network device has active and low power modes. A first voltage regulator regulates supply voltage during the active mode. A second voltage regulator dissipates less power than the first voltage regulator and regulates supply voltage during the low power mode. The MAC device selects the first voltage regulator during the active mode and the second voltage regulator during the low power mode. A crystal oscillator outputs a timing signal to the first PLL during the active mode. A first oscillator selectively generates a first clock signal during the low power mode. The first oscillator dissipates less power than the crystal oscillator.
In wireless networks, there are many reasons that make it difficult to stay in the low power mode for a period of time that is sufficient to significantly reduce average power consumption. For example in an ad-hoc network, each mobile station remains awake after each beacon for a duration of an Announcement Traffic Indication Map (ATIM) window. During the ATIM window, a first mobile station in the ad-hoc network may transmit a directed ATIM message to indicate that it has a message for a second mobile station. Other mobile stations likewise transmit directed ATIM messages if needed. In addition, there may be multicast ATIM messages that need to be sent during the ATIM period. Therefore, all of the mobile stations in the ad-hoc network remain awake during the ATIM window. When a mobile station receives a directed ATIM frame that is addressed to it or a multicast ATIM frame during the ATIM window, the mobile station remains awake for the entire beacon interval.
The relative timing of the directed and multicast ATIM messages during the ATIM window is typically determined using a backoff period. The mobile station counts down the backoff period and then transmits the respective ATIM message (if needed). A random number generator is typically used to generate the backoff period for each mobile station to reduce frame collisions. The use of random backoff periods lengthens the interframe space and increases the time that each mobile station must remain in receive mode. Similarly, a Distributed Coordination Function (DCF) is also implemented after the ATIM window to avoid collisions on the medium. The DCF also employs random backoff periods, which also increases the interframe space.
In some approaches, before the mobile station can enter the low power mode, the mobile station must exchange messages or frames with other mobile stations (hereinafter “power saving frame exchange”). The power saving frame exchange involves data transmission, which is the activity that consumes the most power. Therefore, the power saving frame exchange, which is used each time that the mobile stations enter the low power mode, further increases power consumption of the mobile stations.
In addition, at least one mobile station remains in the active mode between beacon intervals. This is due in part to the fact that at least one mobile station needs to maintain network time. In addition, mobile stations need to complete the power saving frame exchange sequence with another mobile station before going into the low power mode. The last mobile station that is awake does not have another mobile station to communicate with.